linux-kernel - Re: [RFC 2/2] mm: additional checks to page flag set/clear

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Date:	Mon, 30 Dec 2013 14:12:24 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC 2/2] mm: additional checks to page flag set/clear

On Sat, Dec 28, 2013 at 08:45:04PM -0500, Sasha Levin wrote:
> Check if the flag is already set before setting it, and vice versa
> for clearing.
> 
> Obviously setting or clearing a flag twice isn't a problem on it's
> own, but it implies that there's an issue where some piece of code
> assumed an opposite state of the flag.

BUG() is overkill. WARN_ONCE is more then enough.

And I don't think this kind of checks make sense for all flags.

Have you seen any obviously broken case which these checks could catch?

> Signed-off-by: Sasha Levin <sasha.levin@...cle.com>
> ---
>  include/linux/page-flags.h | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index d1fe1a7..36b0bef 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -130,6 +130,12 @@ enum pageflags {
>  
>  #ifndef __GENERATING_BOUNDS_H
>  
> +#ifdef CONFIG_DEBUG_VM_PAGE_FLAGS
> +#define VM_ASSERT_FLAG(assert, page) VM_BUG_ON_PAGE(assert, page)
> +#else
> +#define VM_ASSERT_FLAG(assert, page) do { } while (0)
> +#endif
> +
>  /*
>   * Macros to create function definitions for page flags
>   */
> @@ -139,11 +145,13 @@ static inline int Page##uname(const struct page *page)			\
>  
>  #define SETPAGEFLAG(uname, lname)					\
>  static inline void SetPage##uname(struct page *page)			\
> -			{ set_bit(PG_##lname, &page->flags); }
> +			{ VM_ASSERT_FLAG(Page##uname(page), page);	\
> +			set_bit(PG_##lname, &page->flags); }
>  
>  #define CLEARPAGEFLAG(uname, lname)					\
>  static inline void ClearPage##uname(struct page *page)			\
> -			{ clear_bit(PG_##lname, &page->flags); }
> +			{ VM_ASSERT_FLAG(!Page##uname(page), page);	\
> +			clear_bit(PG_##lname, &page->flags); }
>  
>  #define __SETPAGEFLAG(uname, lname)					\
>  static inline void __SetPage##uname(struct page *page)			\
> -- 
> 1.8.3.2
> 
> --
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-- 
 Kirill A. Shutemov
--
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